Publications of the Astronomical Society of the Pacific 107: 945-958, 1995 October Galaxy Colors in Various Photometric Band Systems M. Fukugita1 Institute for Advanced Study, Princeton, New Jersey 08540 Electronic mail: [email protected] K. Shimasaku2 Princeton University Observatory, Princeton, New Jersey 08544 Electronic mail: [email protected] T. ICHIKAWA Kiso Observatory, University of Tokyo, Kiso-gun, Nagano 397-01, Japan Electronic mail: [email protected] Received 1995 March 10; accepted 1995 June 14 ABSTRACT. A study is made of stellar and galaxy colors using a spectrophotometric synthesis technique. We show that use of good color response functions and a modem determination of the spectroscopic energy distribution for a Lyr gives synthetic stellar colors in a good agreement with photometric observations to about 0.05 mag. The synthetic method then is applied to study galaxy colors using the spectrophotometric atlas of Kennicutt (1992, ApJS, 79, 255), and a comparison is made with observed galaxy colors. The Κ correction is calculated and compared with that of Coleman, Wu, and Weedman (1980, ApJS, 43, 393). We then study colors of galaxies in various photometric band systems and obtain color transformation laws, which enable us to find offsets among different systems. We include 48 photometric bands in our study. 1. INTRODUCTION in addition to a good sample of galaxy SED over a wide range of wavelength. Continual efforts to obtain the SED of Colors of galaxies provide us with valuable information Vega (Oke and Schild 1970; Hayes and Latham 1975; Oke concerning their present-day composition of stars, and ac- and Gunn 1983; Kurucz 1979; Hayes 1985, hereafter re- cordingly it gives us hints about the formation and evolution ferred to as H85; Castelli and Kurucz 1994, hereafter re- of galaxies. While a large number of studies have been made ferred to as CK94) and response functions (e.g.. Code 1960; in this area, different authors have usually used varieties of Αφ 1961; Matthews and Sandage 1963; Azusienis and different photometric band systems. Indeed, each photomet- Straizys 1969; Hayes 1975; Buser 1978; Bessell 1990), how- ric band system has its own advantages. This causes, how- ever, lead us to expect that the error from the first two issues ever, great complications for the analysis of the accumulated can now be controlled to a tolerable level for our purpose. data in a coherent way. To solve this problem, the author We also have a reasonable amount of spectrophotometric usually gives a transformation law from his own photometric data for a few dozens of galaxies. This prompts us to carry system to another, which is more frequently adopted, in out a synthetic study of galaxy colors in various band sys- terms of empirical color-color relations. If we want to trans- tems and to obtain transformation laws among them. With form one magnitude to another, which is not given directly this calculation we can directly estimate the offset between by the author, however, we must combine empirical relations two different magnitude systems, which is important in, for successively for a number of times, which often leads to example, number counts of galaxies. inaccurate results; we occasionally find that the results de- In this paper we carry out a synthetic study of galaxy pend on how these relations are combined. Furthermore, the colors using the best modern data available. In order to see laws given by different authors sometimes disagree, and it is the general accuracy of the method, we first compute broad difficult to trace the origin of discrepancies, since these laws band stellar colors using the SED of stars (Jacoby, Hunter are derived using different standard stars. and Christian 1984; Gunn and Stryker 1982) and compare An alternative way to give transformation laws among them with observed colors (Sec. 1). Such studies have been different colors is to synthetically calculate magnitude using made from time to time by many authors. The new feature of the spectroscopic energy distribution (SED) of galaxies and our calculation is that we use directly the SED of a Lyr to set the filter response functions. Such attempts, however, are not the zero point, rather than resorting to an empirical relation. popular. The reason is that it requires accurate response func- We then calculate broadband colors from SED of galaxies, tions, and also an accurate continuous SED of a Lyr (Vega), comparing them with observations, and brieñy discuss the used to define a zero point of the standard magnitude system. color distribution of galaxies (Sec. 3). With these prepara- tions, we calculate colors of galaxies in various photometric ^Iso at Yukawa Institute, Kyoto University, Kyoto 606 Japan. band systems for galaxies at ζ = 0 and higher redshifts. We 2On leave from Department of Astronomy, University of Tokyo, Tokyo 113, then compute color transformation laws among various pho- Japan. tometric band systems (Sec. 4). 945 ® 1995. Astronomical Society of the Pacific © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 946 FUKUGITA ET AL. 0.5 ι -0.5 üöß -2-10 1 iV-B)oba öO O Fig. 3—As for Fig. 2, but H85 SED is used for α Lyr. (1976) and Buser and Kurucz (1978) have argued that the set of response functions {U3, 82, V) reproduces the relative stellar colors of the Johnson-Morgan photometry most accu- rately. For redder colors we consider both Cousins' system λ (Â) (Cousins 1978) and Johnson's (1965). The response function of the former is taken from Bes sell (1990), which reproduces Fig. 1—Comparison of the H85 SED of a Lyr (dots) with that of CK94 Cousins' photometry most accurately. (solid curve). The wavelength scale is enlarged to emphasize the small dis- Whether the combined use of these response functions crepancy between the two. and the SED of α Lyr provides an accurate zero point is a nontrivial problem, since the SED of α Lyr contains many After completion of this work we find parallel work by strong absorption features whereas the SED was accurately Frei and Gunn (1994), who computed colors and Κ correc- measured only in the continuum part, avoiding absorption tions of galaxies for five photometric systems, using an SED features (e.g., Oke and Schild 1970; Hayes and Latham of Coleman, Wu, and Weedman (1980) with the zero point of 1975; Oke and Gunn 1983). Therefore, in most of the syn- photometric systems calibrated with the SED of F subdwarfs. thetic studies, only relative colors have been considered and Our present study is a substantially more systematic explo- they are converted into observed colors by adding an appro- ration of the subject with a number of checks needed to priate constant, as (B-V)ohs= (ß- y)syn+aß_v etc. An ex- define accurate photometric systems. ception is a synthetic study of Gunn and Stryker (1982), in which a zero point was derived with the aid of AB79 system, by using an SED of F-subdwarf BD+ 17°4708 and three 2. SYNTHETIC STELLAR COLORS FOR THE other F dwarfs, which were calibrated against the absolute STANDARD JOHNSON/COUSINS SYSTEM SED of α Lyr. Since then, much effort has been invested to A number of studies have been done for the standard improve the SED of α Lyr. Here, we use the α Lyr SED to UBV photometric system to give response functions that obtain directly the zero point. The most recent summary of yield stellar colors of standard Johnson-Morgan photometry observed SED has been given by H85, and the most recent (Johnson and Morgan 1953) as accurately as possible. It has work using the stellar atmosphere model has been given by been found (e.g., Hayes 1975; Azusienis and Straizys 1969; CK94. As CK94 have discussed, the agreement between the Buser 1978) that the response function presented by Johnson two seems excellent except for around the Balmer absorption (1965) is not accurate enough to reproduce observed broad- band colors in the standard system. The most accurate re- 0.5 sponse functions known to date are those given by Azusienis and Straizys (1969) for the Β and V bands and by Buser (1978) for the U band. Straizys, Sudzius, and Kuriliene 0.5 I I ? <EEl -0.5 10 11 12 Fig. 2—(a) Difference of synthetic and observed colors Δ(β — V) = (B — V)syn—(Β — V)obs plotted as a function of observed B — V Fig. 4—Difference of synthetic and observed V magnitudes as a function of color. Stellar data are taken from Jacoby et al. (1984) and the a Lyr SED ^Obs- Stellar data are taken from Jacoby et al. (1984) and the α Lyr SED used for this synthetic calculation is CK94. (b) As (a), but for (U — B) color. used for this synthetic calculation is CK94. © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System GALAXY COLORS IN PHOTOMETRIC BAND SYSTEMS 947 Table 1 Synthetic vs. Observed Broad Band Colors of Galaxies galaxy type T(RC3) {U-B]syn {U - B)ohe {B-V)syil {B - Vj^T NGC4649 Ε -5 0.77 1.02 0.97 NGC3379 Ε -5 0.51 0.53 0.90 0.96 NGC4472 Ε -5 0.51 0.55 0.95 0.96 NGC4648 Ε -5 0.51 0.52 0.87 0.92 NGC4889* Ε -4 0.57 0.52 0.84 1.04 NGC3245 SO -2 0.42 0.47 0.91 0.91 -0.5 NGC3941 SO -2 0.44 0.44 0.77 0.91 NGC4262 SO -3 0.43 0.51 0.84 0.94 NGC5866 SO -1 0.38 0.38 0.89 0.85 NGC1357 Sab 2 0.30 0.25 0.76 0.87 Fig.